1. Field of the Invention
The present invention relates generally to heating apparatus and ovens for processing products. More particularly, the present invention relates to ovens which use conveyor systems for processing products by moving a product from an entrance, through the oven, to an exit. The present invention further relates to control systems for controlling conveyorized ovens used to cook food products wherein the temperatures in the oven at which products are being cooked and the time periods during which such products are present and being cooked within the oven are controlled so that when a food product emerges from the conveyorized oven, the food product is properly cooked.
2. Discussion of the Prior Art
Processing of products in a conveyorized oven is a complicated process that may be substantially affected by a number of factors. Processing often consists of more than simply raising a particular product to a specified final temperature. There are many different reactions taking place in a system as complex as food. In food processing, it is desirable to bring the system into a condition that encourages desired reactions to proceed faster than undesired ones. This may require that a food product be maintained at different temperatures for different lengths of time in order to fully complete these various reactions. The requirements of maintaining the product at specified temperatures for certain lengths of time applies to many types of heating and cooling processes, such as heat treating operations, annealing operations and cooking of food products. Different temperatures for varying lengths of time may also be required. Finally, particularly in the case of cooking of food products, there are a number of subjective and objective factors that must be taken into account in order to determine when a particular food product has reached a desired "cooked" or "done" state. For example, before a particular food product may be considered to be "done" or "cooked", the food product may have to have a certain color, consistency, crunch and interior surface or exterior surface temperature.
In order to cook products to the desired "cooked" or "done" state, an oven is used in which heated air is injected into the oven cavity from a suitable heat source via one or more input heat ports, often in the form of hot air jets. A conveyor system is also included which moves the product past each of the hot air jets successively so that when the product exits the oven it is properly "done" or "cooked". In conveyor ovens, the input heat ports may be located in structure known in the art as "fingers", which are positioned at various locations above and below the conveyor. Heated air exits from these heat ports and is directed towards the top and bottom of the product respectively. The hot air fingers are arranged within the oven to provide a predetermined distribution of hot air jets within the oven.
In the prior art, the temperature of the oven cavity is controlled by positioning a temperature sensing element at a selected location within the heated air planium in the oven. In a conventional conveyor oven, the location of the temperature sensing probe is either at or near the entrance end of the oven. The temperature measured by the temperature sensing element is used to thermostatically control the temperature of the oven so that the sensed temperature in the oven can be maintained at a preset temperature value. Temperature control is generally accomplished by turning a heat source (which may be either a gas burner or an electric heater) on to increase the temperature of the oven or off to allow a decrease in the temperature of the oven.
The cooking time for a particular product, that is, the time which the product has to remain in the oven in order to reach the proper "done" or "cooked" state, depends on the temperature of the oven, the type of product which is being cooked, and the degree of cooking required in order to reach the "done" condition. With a conveyor type oven, assuming the temperature of the oven remains constant at a preset temperature, the cooking time is determined by the speed of the conveyor as it moves the product through the oven. Under normal circumstances, the user of a conveyor oven sets the oven at the desired preselected temperature, and then sets the conveyor speed so that the food product remains in the oven, i.e., the product proceeds from the entrance to the exit in the preselected cooking time. The conveyor remains at the selected constant speed so that the food product moves through the oven for the preset cooking time.
There are several problems with this type of prior art conveyorized oven. These problems fall mainly into two categories: oven temperature control, particularly due to variations in the load placement through the oven, and conveyor speed control.
The prior art thermostatic temperature control system is generally not able to maintain the oven temperature at the preset temperature for several reasons. First, the placement of the thermostatic temperature control cannot sense the uneven loading of the oven. Second, the thermodynamics of the oven result in uneven temperatures due to the uneven loading Second, as noted earlier, the oven itself may have some hysteresis with respect to temperature changes within the oven cavity due to thermal inertia and thermal mass of the oven components themselves.
In some cases, the heat or air distribution within the oven may be purposely selected to be non uniform. For example, in a conveyor oven, a greater number of hot air jets may be desired in the first half, or entrance zone of the oven and a lesser number of hot air jets may desired in the second half, or exit zone of the oven. The input hot air jets can be appropriately arranged in the oven so as to provide the desired non-uniform air distribution profile by locating additional "fingers" in the entrance zone of the oven. Because of variations in the airflow which occur and because only a single temperature sensor, which is normally located at the entrance end of the oven is used, a desired non-uniform temperature profile results.
The non uniform temperature profile is affected by changes in the heat load placed on the oven due to the quantity and types of products inserted at the entrance end of the oven. Furthermore, since conventional ovens generally have only a single temperature sensor located in the entrance zone of the oven, temperature control of the oven cannot follow the load as it is moved through the oven. The effect of such load changes and temperature control produces adverse results in the final cooked product at the exit zone of the oven. For example, suppose a product representing a relatively heavy thermal load is inserted at the entrance end of the oven, thereby requiring a higher energy in the entrance zone than would be required for more even or idling thermal loads. In order to compensate for the high thermal load, the control system will add more energy to the heated air being injected into the entrance zone which will result in a corresponding rise in the temperature of the heated air being injected into the exit zone because there is a light load in the exit zone. A product that was moving through the oven and is in the exit zone when this temperature rise occurs may, as a result of this increase in temperature, become overcooked or over browned. Conversely, when a product is located in the exit zone of the oven and no product is located in the entrance zone, the oven will reduce the energy supplied to the oven cavity despite the thermal load in the exit zone. Therefore, the product may emerge undercooked.
A fourth problem with this prior art temperature control system is that the thermal load on the oven may adversely affect the degree to which food products are cooked. For example, if food products representing a very high thermal load were placed on the conveyor which moves through the oven at a constant speed, and in combination with the lag time of the oven, the temperature control system may not be able to compensate initially for the high thermal load and consequently, the food products would emerge at the end of the conveyor from the oven in an undercooked state.
There is another set of problems with prior art conveyorized ovens and control systems due to the control of the conveyor system. A first problem occurs because the conveyor speed does not change with different products having different cooking time requirements. The conveyor speed is generally manually set by the operator to a fixed speed. Therefore, with the oven set at a fixed temperature, and the conveyor speed set so that a product moves through the oven in a particular time, products which require less than the preset time for proper cooking will be over done while products which require more time or a higher temperature than the preset time and temperature will be underdone.
Another problem occurs in other prior art oven conveyor systems in which the conveyor speed through the oven cannot be adjusted and is constant and independent of the temperature of the oven. If the oven temperature should happen to be low, a product will not spend enough time in the oven to be fully cooked and consequently will emerge in an undercooked state. On the other hand, if the oven temperature is too high, a product may remain in the oven too long and emerge at the exit end of the oven in an overcooked state.
Moreover, not only are these prior art conveyorized ovens unable to be adequately controlled to compensate for variations in cooking temperatures and heat distribution that occur during the cooking process for a particular product, they are unable to compensate for the variations in the cooking process required when the oven is to be used for cooking different types of products. This problem is particularly acute when different types of products are cooked at the same time. The oven is unable to compensate for different products which react with different sensitivities to temperature variations within the oven. This problem is especially aggravated when cooking different types of food products in conveyorized ovens.
Further, conveyor cooking devices rely on the length of the contained device to increase the throughput. Conveyor cooking devices tend to be large in order to accomplish high throughput. The smaller to normal size devices have two zones that require separate sets of sensors in order to measure the temperature. Longer conveyor cooking devices have multiple heat source and more zones that need to be accounted for in order to accurately control cooking. In these conventional ovens, a single heat sensing means placed within the oven chamber does not give enough information about the temperature of the impingement or convected air or enough information about the energy removed from that air on average within a zone. Consequently, accurate sensing and control of the oven temperature and resultant cooking times is difficult. Furthermore, infrared type ovens only control the temperature of the radiant element and do not take into account ambient temperature changes due to variation of the load, thus further contributing to inaccurate cooking.
Therefore, an object of the present invention is to provide a method and apparatus for cooking products of differing types or cooking times so that all products, when cooked in the oven are cooked to the proper "done" or "cooked" state.
Another object of the present invention is to provide a method and apparatus that accurately compensates for changes in oven temperature during the cooking process.
Another object of the present invention is to provide a method and apparatus that compensates for temperature fluctuations and accurately controls the cooking time of a particular product.
Another object of the present invention is to provide a method and apparatus that accurately compensates the cooking time of a particular product as a function of the oven temperature and the sensitivity of the particular product to changes in oven temperature.
Another object of the present invention is to provide a method and apparatus for cooking food products of differing types so that all food products of the same type are uniformly cooked to the same "cooked" state.
Still another object of the present invention is to provide a method and apparatus for uniformly cooking different food products that may be operated by people having a wide range of skill levels.
Another object of the present invention is to provide a method and apparatus that compensates the temperature control due to load variations within the oven.